In the preparation of cellulosic pulp for use in the manufacture of paper, a common process includes the digesting of wood chips in pulping liquor to break down the pulp into individual fibers and bunches of fibers by dissolving the substances, such as lignins, which bind the fibers together. The spent pulping liquor will therefore contain such dissolved substances and spent chemicals, and the next stage in the preparation of the pulp is a washing stage for the purpose of separating the fibers from the liquor, and also of recovering whatever products of value remain in the liquor.
The art has proposed a variety of types of washers for use in this stage of the preparation of paper making fiber. One type of such apparatus is a flat bed washer which is generally similar in construction and mode of operation to a Fourdrinier paper machine, in that it incorporates an endless foraminous belt ("wire"), a headbox which delivers the pulp suspension in liquor to one end of the horizontally traveling upper run of the wire, successive washing zones along the length of this run, and means at the downstream end of the run for receiving and removing the resulting washed pulp. Pulp washers of this type manufactured by the assignee of the present invention in accordance with Ericsson U.S. Pat. No. 4,154,644 of 1979 have been notably successful, and the present invention was developed to improve the operation and results obtained by such pulp washers.
In the operation of a pulp washer of the Ericsson patent type, the suspension of digested pulp from the digesting system is diluted to a sufficiently low consistency, e.g. 1.5 to 3%, and deposited on the upstream end of the wire run where a mat is formed as the liquid drains through the wire and is recycled to dilute more of the suspension to be washed. Commonly the solids content of this mat is of the order of 8 to 10% at the end of the initial drainage step.
The remainder of the wire run downstream from the mat-formation zone is divided into a series of washing zones to which washing liquid is supplied from above for drainage through the mat and the wire. Fresh washing liquid is supplied to the last of these washing zones, at the downstream end of the wire run, the liquid drained from that last zone is collected and delivered to the washing zone immediately upstream from the final zone, and these steps are repeated for each of the other zones to effect countercurrent washing of the pulp mat as it progresses from the formation zone to the discharge end of the washer, while the filtrate from the first washing zone may be sent to an evaporator station for removing of its dissolved constituents.
The operation of a pulp washer of this type may therefore be described as being according to the displacement washing principle. That is to say, once the pulp mat has been formed, it is not rediluted but simply is subjected to repeated washings by application on top of the mat of washing liquid with the liquid applied in each washing zone having a lower concentration of liquor than the filtrate from the preceding zone. The liquid applied in each zone enters the mat substantially en masse and thereby displaces the liquid which was carried into the zone in the mat and causes it to drain therefrom through the wire.
Among the mechanical elements of a washer according to the Ericsson patent is a hood which encloses the entire apparatus downstream from the headbox, and a series of receptacles below the operating run of the wire and in sealed relation with this hood. In operation, vacuum is applied to these receptacles, and/or gas pressure is developed within the hood, to augment the action of gravity in forcing the washing liquid through the pulp mat on the wire, and one of the features disclosed in the Ericsson patent in the recycling of gases and vapors drawn through the wire into the upper spaces in the receptacles back to the hood to increase the pressure differential above and below the wire.